AAA Battery Charger

[smartsarath2003]

Hai,
I want to make a small AAA battery charger. I have an idea about a const current charger using LM317. So far I decided to use a LM317 and a 555 timer .
Could any one please let me know any ideas without using microcontrollers. :

Thanks

 

[ee]

you could make a simple one with a rectifier and resistor(s)

 

[smartsarath2003]

I also want to implement a cycle Charge-rest-discharge-rest-charge
I red somwhere that this(pulse-charge) will be good method ratherthan continuous charging. The discharge cycle involves sensing the voltage of the battery by connecting a load thus by finding the battery is fully charged/not. When it is fully charged , I can disconnect the circuit. If am also thinking about using thermistors to find the temp of the battery to check the battery charged or not. But don't have any clue how I am going to implement all these without a microcontroller
Thanks

 

[audioguru2]

Hi Smart,
Why not use a battery charger IC? www.maximic.com have some nice ones.
Read about rechargeable batteries in our Articles section, and also at www.energizer.com .

 

[MP1]

Maxim does have some nice battery charger chips. They run a little more than $2 each to buy direct from Maxim. You might have a hard time finding the DIP package. SOIC is the most popular. The nice part of using Maxim's charging ICs is that they have plenty of supporting documentation for their products. You can probably even find the schematic you want to use.
...Another plus: Maxim is great at sending samples. If you only want one or two, you are set.

MP

 

[smartsarath2003]

Hai,
Thanks for the reply
By making the charger,Iam trying to learn things. So I want to start from basics, rather than using integrated IC's.....
Any ideas

 

[Alun]

I would still go for the Maxim chip if I were you, as you can still learn a lot from the supporting documentation for their products. Building a smart charger from scratch is not a good idea and if you make a mistake the batteries could explode.

 

[smartsarath2003]

Hai, here is my simple pulse charger. Applies 125mA charge for approximately 500mSec and no charge for 100mSec. But still to include disconnecting ckt after complete charge. I am thinking of using a thermister, to see the battery is hot enough or not and then disconnecting the charging ckt. But surprisingly the battery didn't get hot when i touched at the end. I left the charger on for about 3.5hrs(7hrs-50mA given on the 250mAh battery). I have an idea of using a transister to connect across the Capacitor of 555 timer and activate it when the battery is fully charged. Any other better ideas

Thanks

View attachment 36692

 

[audioguru2]

Hi Smart,
Haven't you read about battery charging safety in Maxim's and Energizer's notes?
Do you know how a battery's temperature is delayed on its outside from its temperature rise on its inside?
Do you know at what temperature a battery will explode?

Battery and charger manufacturers use temperature sensing only as a backup to their main charge-sensing and timing methods in case the main electronics fails.

I think you are lucky the battery didn't get hot and explode. You are lucky because the operation of your 555 is backwards to what you want. Its output is high for a few hundred ms then low for almost no time. Therefore the LM317 current-source and the battery's charging current are active for almost no time.

Why are you using a power transistor to pull down the fairly low power of the LM317's output? ;D

 

[Alun]

If you make the following modificaitions your circuit will be a lot better.

Swap the 55K and 19K resistors round.

Use a smaller transistor like a BC547 and connecet it in series with the battery, collector to the cathode and emitter to 0V.

Use a bigger base resistor like 4.7K

 

[smartsarath2003]

Haven't you read about battery charging safety in Maxim's and Energizer's notes?
Do you know how a battery's temperature is delayed on its outside from its temperature rise on its inside?
Do you know at what temperature a battery will explode?

Battery and charger manufacturers use temperature sensing only as a backup to their main charge-sensing and timing methods in case the main electronics fails.

I think you are lucky the battery didn't get hot and explode. You are lucky because the operation of your 555 is backwards to what you want. Its output is high for a few hundred ms then low for almost no time. Therefore the LM317 current-source and the battery's charging current are active for almost no time.

;D

Hai,
I decided to use temparature sensing to check battery is charged or not after reading energiser manual for NiMh and said thats mostly same for NiCd as well. The document is avilable at http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf
They boasted that temp sensing is better and accurate way. The other methods infront of me is simple DC loading or complex AC impedance. If you have good idea plz sujjest me a simple and good method
Also you said LM317 current source and battery's charging current are active for almost no time.
What does that mean? The 555 timer just disconnects source for around 120msec and connects for 550msec. I done that way because I red in another article that its not good for battery to have continuous charge/discharge. So I gone for pulse charge.
BD677 was used only in case I increase the charge current.
any more comments/Help ???
Thanks

 

[audioguru2]

Also you said LM317 current source and battery's charging current are active for almost no time.
What does that mean? The 555 timer just disconnects source for around 120msec and connects for 550msec.

Hi Smart,
Sorry, I was mistaken when I said that. I thought the diode was with the opposite polarity.

The 555 charges the capacitor through the 55K resistor in series with the diode for about 550ms. The cap is discharged by the 19K resistor for 209ms.  I don't know how you figured a discharge time of only 120ms.
Since the output of the 555 is high while the cap is charging, and the transistor is turned on disabling the LM317 current regulator, the battery is charged for 550ms then a pause of 209ms.

Temperature rise sensing when the charging enters overcharge depends entirely on extremely good thermal contact of the sensor with the battery. It might be difficult to have good thermal contact but still allow freedom to remove and insert battery cells easily.
You should have some backup methods to stop the charging if the sensor or its thermal contact fails. A timer would help but isn't good enough as a backup because a fully-charged battery might be attempted to be charged by mistake. Then you know what will happen:
Ka-boom!

Your battery didn't get hot when you charged it for 3.5 hours because it is fully-charged when about 140% of its discharge capacity is used for charging. The extra 40% is due to charging inefficiencies like heating.  ;D ;D

 

[smartsarath2003]

Your battery didn't get hot when you charged it for 3.5 hours because it is fully-charged when about 140% of its discharge capacity is used for charging. The extra 40% is due to charging inefficiencies like heating.

 

[audioguru2]

Hi Smart,
The battery cell capacity is 250mA/hrs. The recommended charge is 7 hrs at 50mA, which is 350ma/hrs. 350/250 = 140%. You charge for 40 percent more than its capacity.

The air inside the battery compartment won't heat like the battery unless there is very little air and it is sealed very well. You need good thermal contact from the temp sensor to the battery cell. Even then it will be difficult to determine at which temp the battery is fully charged.

Memory effect happens with Ni-Cad, not Ni-MH. They are different when charging too. A Ni-MH keeps heating more and more as it is being charged, a Ni-Cad doesn't heat much until it is fully charged.

You still might have venting or an explosion, look at the pressure rise much sooner than the temp for these curves that are for Ni-MH, not Ni-Cad: 

View attachment 36701